As the author proposes to spar with the lighter guys, so he will work more in defense and movement. Extra weight will force him to faster contract muscles during dips / waves / bobs / slips, also endure fatigue due to movement.

No it will change his muscle activation strategies, he'll have to contract sooner, at a different sequential pattern and tempo, with more force. As we all know about muscle's force-velocity characteristics we know that the more force the slower the movements have to be. As we all know the brain adapts to how we train. As we all know, fatigue effects speed and coordination negatively.
So we should all know that wearing a weight vest will make us slower, **** up our punching biomechanics and make us tired sooner so we get less quality work in. But hey, this is boxing and we do dumb **** just for the hell of it.